In recent years, plasma processing technologies have been applied not only in the conventional fields of semiconductors but also a wider range of technical fields, including the fields relating to nanotechnologies, such as optical devices (e.g. semiconductor lasers or diodes), microelectromechanical systems (MEMS; e.g. gyroscopes or sensors) and carbon nanotubes, as well as medical and biotechnological applications (e.g. micro knives or sterilization).
The expansion of the application fields has also been accompanied by a growing demand for higher productivity, which has naturally led to a request for improving the efficiency of the plasma processing apparatus. The efficiency can be improved in several ways, e.g. by increasing the size of the sample (substrate or wafer), enhancing the processing rate, or accurately controlling the shape. To meet these requirements, inductively coupled plasma (ICP) systems have been improved in various aspects, such as the use of an enlarged induction coil or parallel coils. Among those various requirements, the most critical requirement for the currently used plasma processing apparatuses is the creation of a uniform plasma capable of processing larger samples.
In a plasma processing apparatus, supplying an electric current into the induction coil induces a magnetic field. The potential difference given to the coil collaterally creates an electrostatic potential distribution around the coil, and this potential stays in the vacuum space. The electrons contained in the plasma have the effect of forming a shield against an externally given potential and therefore gather at high-potential portions of a dielectric plate separating the coil from the plasma, causing the potential of those portions to be lowered. The lowered potential creates a secondary factor for attracting positive ions, causing the sputtering of the dielectric plate.
Since the potential created along the coil lacks symmetry, the spatial distribution of the plasma composition will be asymmetrical. With such a plasma, it is difficult to achieve a high level of uniformity when processing the substrate. Such a problem of the collateral potential produced by the induction coil has been known as the fringe effect of the coil. This effect is regarded as a critical problem that can affect the performance of the device to be manufactured with the plasma processing apparatus.
One conventional method for canceling the fringe effect of the coil is to use an electrostatic shield called a Faraday shield. It is a thin sheet of conductive metal, which is to be set between the plasma and the induction coil, i.e. next to the dielectric plate. In a plasma processing apparatus having a Faraday shield, when an electric current is supplied to the induction coil, the magnetic field thereby induced can pass through the Faraday shield and penetrate into the plasma, whereas the potential will be blocked by the Faraday shield and cans of reach the plasma.
Unfortunately, the Faraday shield has the problem that eddy current is generated in the shield material, causing a loss of energy supplied from the induction coil. To prevent this phenomenon, a large number of small windows are created in the shield material to cut the passage of the eddy current. However, there still remains the problem that the shielding effect and the power loss are in a trade-off relationship. Additionally, it requires a cumbersome and time-consuming process to optimize the opening ratio and shape of the small windows.
Another approach to canceling the fringe effect is to use a commonly known coil especially developed for reducing the fringe effect. For example, the induction coil disclosed in Patent Document 1 consists of two or more identically shaped coil elements connected electrically in parallel. The coil elements are arranged so that their center coincides with the center of the object to be processed, and their input ends are located at angular intervals equal to 360° divided by the number of the coil elements (i.e. at equal intervals around the center). The coil elements are three-dimensionally formed on the surface of a ring having an arbitrary sectional shape, with each coil element displaced from the other in both radial and vertical directions. Patent Document 1 states that such a coil configuration effectively improves the uniformity of the plasma in the circumferential direction of the coil.
Another example of the induction coil developed for reducing the fringe effect is disclosed in Patent Document 2. The induction coil disclosed in this document has a feed end to which a radio-frequency (RF) power is supplied and a ground end which is connected to ground, and includes at least two loop antennas provided electrically in parallel. The feed ends and ground ends of these loop antennas are symmetrically arranged with respect to the center of the same antennas. Each loop antenna is held parallel to and partially intersecting with the other antenna or antennas, with its feed end and ground end located farther from the chamber and its middle section closer to the chamber.
Patent Document 2 states that using the induction coil having the previously described configuration enables the voltage to be uniformly distributed over the entire induction coil and thereby produce a uniform and symmetrical plasma density profile in the rotational direction within the chamber.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-303053 (FIG. 6)
Patent Document 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-537839 (FIG. 3)